1. Field of the Invention
The present invention relates to a duplexer for use in a microwave band, for example, and a communication apparatus.
2. Description of the Related Art
A transmission frequency band required for the transmission side circuit of a duplexer for use in PCS is 1850-1910 MHz, and a reception frequency band for a reception side circuit is 1930-1990 MHz. It is necessary for both of the transmission side circuit and reception side circuit to have a wide pass-band of 60 MHz. On the other hand, the separation assured to separate the transmission frequency band from the reception frequency band is 20 MHz. That is, the separation between the both bands is very narrow.
Further, the duplexer composes the phase of the transmission side circuit and that of the reception side circuit. In the case of PCS, the phase of the transmission side circuit and that of the reception side circuit are ideally composed by setting the transmission side circuit to have a high impedance (open) in the reception frequency band of 1930-1990 MHz, and setting the reception side circuit to have a high impedance (open) in the transmission frequency band of 1850-1910 MHz.
FIG. 8 shows an example of the circuit configuration of a prior art duplexer 1. In the case of a PCS system, the separation between the transmission frequency band and reception frequency band is narrow, namely, 20 MHz. Accordingly, the transmission frequency band is divided into two ranges of 1850-1880 MHz and 1880-1910 MHz, and also, the reception frequency band is divided into two ranges of 1930-1960 MHz and 1960-1990 MHz. That is, the frequency bands become narrow, and the separations are wide. In particular, reactance elements (PIN diode) are connected to resonators, respectively, and control the voltages of the resonators, so that the two types of pass-bands of each of the transmission side circuit 25 and the reception side circuit 26 can be changed over, resulting in reduction of the number of the filter stages. Like this, it is attempted to downsize the duplexer and give high qualities thereto. In FIG. 8, a transmission terminal is designated by Tx, a reception terminal by Rx, an antenna terminal by ANT, resonators in the transmission side circuit 25 by 2 and 3, resonators in the reception side circuit 26 by 4 to 6, coupling coils by L1 and L11, coupling capacitors for determining a rejection-band attenuation by C1 and C2, capacitors by C5, C6, and C24, frequency band variable capacitors by C3, C4, and C7 to 9, PIN diodes by D2 to D6, choke coils by L2, L3, and L6 to 8, control voltage supply resistances and capacitors by R1 and R2, and C22 and C23, respectively, coils and capacitors constituting phase circuits by L20 and L21, and C15, respectively, and coupling capacitors by C11 to C14.
CONT1 designates a voltage control terminal for controlling the voltages of the PIN diodes D2 and D3 of the transmission circuit 25, and CONT2 a voltage control terminal for controlling the voltages of the PIN diodes D4 to D6. When positive voltages are applied to the voltage control terminals CONT1 and CONT2, the PIN diodes D2 to D6 are in the on state, and the duplexer 1 operates through the LOW channel. That is, as shown in FIG. 9, the pass-band of the transmission side circuit 25 becomes 1850-1880 MHz, and that of the reception side circuit 26 becomes 1930-1960 MHz. To the contrary, when the control voltages are zero with no voltages being applied to the voltage control terminals CONT1 and CONT2, the PIN diodes D2 to D6 turn off, and the duplexer 1 operates through the HIGH channel. That is, as shown in FIG. 9, the pass-band of the transmission side circuit 25 becomes 1880-1910 MHz, and that of the reception side circuit 26 becomes 1960-1990 MHz.
A portable telephone is put on standby for a reception wave except the time when speech is carried out. In case the frequency during the reception wave standby is 1930 MHz and the reception wave standby is carried out with positive voltages being applied to the voltage control terminals CONT1 and CONT2, the battery of the portable telephone is quickly exhausted, which causes the problem that the reception wave standby time becomes short.
It may be supposed that as countermeasures against the problem, the control voltage of the voltage control terminal CONT1 is set at 0V and a positive voltage is applied to the voltage control terminal CONT2 only. Since a consumption current flows through only the reception side circuit 26 during the reception wave standby, the exhaustion of the battery can be suppressed. However, as to a system such as PCS in which the frequency of the transmission frequency band is lower than that of the reception frequency band, the separation between the pass-band (1880-1910 MHz) of the transmission side circuit 25 and that (1930-1960 MHz) of the reception side circuit 26 is very narrow, as shown in FIG. 10, when the PIN diodes D2 and D3 in the transmission side circuit 25 is turned off (in the off state) and the PIN diodes D4 to D6 in the reception side circuit 26 is turned on (in the on state). Therefore, it is difficult to set the transmission side circuit 25 to have a high impedance (open) in the reception frequency band of 1930-1960 MHz. Thus, there arises the in problem that the insertion loss of the reception side circuit 26 is large.
FIG. 11 is a graph showing the measurement results of the band-pass characteristic S32 and reflection characteristic S22 (see FIG. 8) of the reception side circuit 26 obtained when positive voltages are applied to the voltage control terminals CONT1 and CONT 2. In this case, the insertion loss of the reception side circuit 26 was 3.3 dB. On the other hand, FIG. 12 is a graph showing the measurement results of the band-pass characteristic S32 and reflection characteristic S22 of the reception side circuit 26 obtained when a positive voltage is applied to the voltage control terminal CONT 2 only. In FIG. 12, the waveform is distorted in the part thereof shown by a circle A. In this case, the insertion loss of the reception side circuit 26 was deteriorated to be 5.0 dB.
To overcome the above described problems, preferred embodiments of the present invention provide a duplexer of which the consumption current is small and the insertion loss is low, and a communication apparatus.
One preferred embodiment of the present invention provides A duplexer comprising: a first external terminal; a second external terminal; an antenna terminal; a first frequency variable filter electrically connected between the first external terminal and the antenna terminal, and composed of at least one resonator and a reactance element electrically connected to the resonator and capable of being voltage-controlled; a second frequency variable filter electrically connected between the second external terminal and the antenna terminal, and composed of at least one resonator and a reactance element electrically connected to the resonator and capable of being voltage-controlled; the predetermined reactance element of the first frequency variable filter being in the on state when the reactance element of the second frequency variable filter is in the on state.
Hereupon, the first frequency variable filter is a transmission filter, for example, and the second frequency variable filter is a reception filter, for example. As the reactance elements, for example, PIN diodes and variable capacitance diodes are used.
When the reactance element of the second frequency variable filter is in the on state, the predetermined reactance element of the first frequency variable filter is in the on state. Thereby, the impedance of the first frequency variable filter is enhanced in the resonant frequency band of the second frequency variable filter. Accordingly, the insertion loss of the second frequency variable filter is suppressed. In addition, since only the predetermined reactance element of the first frequency variable filter is in the on state, the current consumption is reduced as compared with the case where all the reactance elements of the first frequency variable filter are in the on state. Thus, the power consumption during reception wave standby is decreased.
Another preferred embodiment of the present invention provides a communication apparatus including any one of the duplexers described above. Accordingly, the power consumption during reception wave standby is suppressed, and the loss of the reception side circuit is reduced.